The present invention relates to the processing of meat for tenderization and/or the killing of bacteria in the meat, by subjecting the meat to shock waves which are plasma waves or pulses generated by capacitive discharge between two electrodes.
Meat can be tenderized and at least partially sterilized by shock waves, i.e. acoustic or pressure pulses, from explosions caused by a chemical explosive charges or a capacitive discharge between two electrodes, such as shown in the U.S. Pat. Nos. 5,273,766; 5,328,403; 6,120,818 and 6,168,814 B1 in the name of John Long, and U.S. Pat. No. 6,224,476 B1 in the name of Long et al. A shock wave travels outwardly from the explosion site at the speed of sound, or somewhat higher in the case of high-intensity shock waves; and, like an audible sound echoing from a wall, will reflect from a shock-wave reflective surface.
The condition for reflection of a shock wave is that the speed of sound, which varies depending on the medium through which it travels, changes at an interface between two different media. A pressure wave travels in water at about 1500 meters per second, faster than its rate of travel through air; the same wave travels in stainless steel at 5800 meters per second, nearly four times faster than its rate through water. This difference in the speed of sound is close to the difference in speed for shock waves, which are basically high pressure sound waves; they propagate by the same mechanism as sound does, but are sharp pulses and typically have a much higher sound intensity or pressure rise (sometimes called xe2x80x9coverpressurexe2x80x9d) than most sounds.
When a sound or shock wave in water encounters a steel surface, most of the wave is reflected away from the surface because of the difference in speed (also referred to as an xe2x80x9cacoustic impedance mis-matchxe2x80x9d), with only a small portion passing into the steel. In some of the aforementioned related patents, the reflection of shock waves from a thick steel surface was used to increase the intensity of the shock pulse. The pulse of the shock waves from an explosion is brief but has an appreciable length, and when the pulse is reflected from steel it passes through itself, increasing the shock wave pulse intensity. (The same effect is seen at a seawall, where ocean waves reflecting from the wall splash to a greater height up the wall than they reach in open water.)
In a preferred embodiment according to Long ""766 and ""403, the meat was placed in plastic bags which were lined along the bottom of a hemispherical steel shell, the shell was filled with water, and an explosion was set off in the geometrical center. The shock wave traveled outwardly to reach all the meat at roughly the same time and hit the meat with roughly the same overpressure or shock wave intensity, passing through the packaging film and meat twice due to the reflection from the steel shell. (The meat and the enclosing bags, having an acoustic or mechanical impedance close to that of water, do not appreciably reflect the shock pulse.)
This earlier embodiment works very well in tenderizing and at least partly sterilizing the meat lined along and adjacent the inner wall of the shell, but it has some drawbacks. Importantly, this embodiment is inherently a batch operation, and the equipment is expensive. A stainless steel hemisphere four feet in diameter and two inches thick is not inexpensive, and the equipment needed for moving blast shields, water changers, and so on is complex and costly. Packing and removing the meat is slow, and further delays are mandated by safety concerns; workers should not load the hemisphere while the explosive is rigged, for example.
Another drawback is that the water is blown upwardly out of the hemispherical shell by the explosion and must be replenished. In the case of chemical explosives, it is preferable to drain off any remaining water and replace it with fresh water which is untainted by chemical by-products of the explosion, even though such water does not even come directly into contact with the meat. This draining and replenishing takes time and uses a great deal of water.
Also, the explosive force in the aforementioned embodiment is not balanced. The geyser of blast gases, steam, and spray out the top of the hemisphere causes a large reaction force which drives the hemisphere downwardly, and this must be resisted by large springs, dashpots, and so on, this additional equipment also being expensive and tending to deteriorate too quickly. A special blast-shield dome above the shell as in Long U.S. Pat. No. 5,841,056 is needed to absorb the force of the geyser.
placing the meat into protective plastic bags causes additional problems as well, and is preferably avoided.
The placement of the meat against or in near adjacency to the surface of the shock-wave reflective steel is the root of some of the difficulties with previous embodiments as discussed above, and such placement has limitations which prevent any substantial improvement. The width of the layer of meat which can be tenderized is limited by the duration of the shock pulse, because if all the meat is to be subjected to intensity doubling then the thickness of the shock pulse must be at least twice the thickness of the meat, so that the pulse intensity will be doubled throughout the thickness of the meat. If the pulse is of very short duration, its trailing edge will have passed into the meat layer just as the leading edge is reflecting from the steel, and only the portion of meat closest to the steel will experience the doubled shock intensity; the rest will undergo two passes of the non-doubled shock wave. The width of the shock pulse in meters is roughly 1500 m/s divided by the pulse duration in seconds.
Limiting the thickness of meat means that the size of the hemisphere must be increased if each batch of meat to be treated is to be large enough that the overall processing rate is not too slow. But increasing the hemisphere diameter means that the shock pulse will be weaker, since the pressure intensity of a spherical wave falls off approximately as the cube of the radius (which corresponds to the distance from the source or sources of the explosion).
If the intensity doubling of the earlier embodiments were not insisted on, then the layer of meat could be spaced further away from the shock-wave reflective inner surface of the hemispherical shell, and the greater intensity of the shock wave would make up for the intensity doubling. If the meat were moved inwardly by about 29% of the hemisphere radius (precisely, 1.000 minus 0.707) then the single-pass shock wave intensity would be just as great as the doubled intensity at the inner surface of the hemisphere, even if the explosion energy were not increased. (The shock wave would pass outwardly through the meat and then, after reflection from the steel surface, pass back inwardly through the meat.) This shows that placing the meat directly against or closely adjacent a reflective surface is not essential.
However, the problem then arises as to how the meat can be supported against moving away from the explosion. Such problem is solved in the aforementioned Long U.S. Pat. No. 6,168,814 B1 by making the container xe2x80x9cacoustically transparentxe2x80x9d so that the shock wave will pass through the container without being significantly diverted in direction or delayed in passage.
There are several ways to make a container acoustically transparent. One is make the container of wires, which sound (and a shock wave) can pass around, but a wire container will not in all cases adequately support the meat; and, depending on the size of the wires or rods from which it is formed, will interfere with the shock wave. A preferred way, though, is to make the container of a material having roughly the same xe2x80x9cacoustic impedancexe2x80x9d as the liquid in which it is immersed. If the impedances of the container material and the liquid are about the same, then the shock wave will have the about the same speed in both materials. According to Huygens"" principle, the waves then will not be bent by refraction. Neither will they reflect from the interface between the liquid and container material.
(An analogy can be made to light waves. If a solid object immersed in water has an xe2x80x9cindex of refractionxe2x80x9d (optical impedance) close to that of the water, it will be nearly invisible because the light rays passing through it will not bend. For example, a piece of clear ice or glass is less visible in water than in air, because there is little difference between the indices of refraction.)
If the liquid is water as is preferred, the container may be made of a material in which the speed of sound is similar. Such materials are available. In gum rubber, for example, the speed of sound is only 3% higher than in water, and several more durable plastics are close enough in their acoustic impedances to water that they are quite suitable for the meat container. One suitable and well-known material, which is approved for use with food, is TYGON, which is a plasticized vinyl polymer; others are polyethylene and polypropylene. Other plastics can be routinely tested for acoustic transparency and durability in the explosive environment. If a hemispherical meat container made of TYGON or the like were suspended concentrically inside the hemispherical shell, the meat could be tenderized without the need for reflection, as discussed above.
But this would not eliminate all the problems with the earlier embodiments, namely the need for batch processing and the associated slowness and complex equipment. In order to attain either continuous processing, semi-continuous or intermittent processing, or improved batch processing, the later embodiments exchanged the earlier hemispherical geometry for an essentially cylindrical geometry, while in some embodiments the batch container was exchanged for a conduit (e.g. a TYGON tube) through which the meat product is pumped or carried in the case of hamburger or the like (i.e. a slurry) or by flowing water in the case of pieces of meat, e.g. de-boned chicken parts or plastic film wrapped beef. The advantages of a solid pipe of suitable-impedance plastic, substantially transparent to the shock wave, as compared to a conduit made of fine mesh, are evident in relation to food transport; such a tube is also more xe2x80x9ctransparentxe2x80x9d to shock waves than is a mesh or framework. TYGON, and other suitable plastics, are available in the form of tubing.
Consequently, in Long U.S. Pat. No. 6,168,814 B1, a hollow and roughly cylindrical shock reflector surrounds the plastic conduit or static meat holder so that the shock waves are internally reflected. Even if the geometry is not so precise that shock wave reflections are perfectly arrayed, the reflector serves as a reverberant chamber in which the many shock wave echoes produce a quasi-hydrostatic pressure pulse.
As the meat is pumped through the plastic conduit in the case of such a continuous system, explosions are set off near the conduit repeatedly, at short enough intervals so that all of the meat passing through the conduit is exposed to shock wave treatment. All reflections of shock waves are preferably from surfaces at a distance from the plastic conduit and the meat.
The meat in such a continuous process is preferably subjected to a plurality of shock wave passages in short succession, which create the quasi-hydrostatic pressure wave effect of overlapping pulses, either through overlapping of the shock waves and a consequent increase of the shock intensity, or by failure of the meat or bacteria therein to xe2x80x9crecoverxe2x80x9d from one shock before the next shock quickly arrives. The shock waves may impinge on the meat either directly, by reflection, or after plural reflections from a number of surface areas of the reverberant cylindrical chamber.
From a single explosion generated from either a chemical explosive or a capacitive discharge, a spherical shock wave expands rapidly and uniformly until it encounters a change in acoustic impedance and is reflected or refracted. With a proper arrangement of reflective surfaces the expanding spherical shock wave from the single explosion can be diverted and reflected so that the reflections impinge on the meat in the conduit from several directions in a short time.
If the xe2x80x9craysxe2x80x9d (portions of the wave front traveling perpendicular to the wave front surface) all travel the same distance to reach the conduit, then the waves will impinge on the meat inside the conduit simultaneously.
While the prior embodiments of Long including those mentioned above work very well, further improvements have been achieved according to the present invention, including the provision of less costly equipment, improved efficiency, and greater effectiveness. These improvements have been brought about by a number of changes, each one of which provides a degree of improvement, and which in combination provide very significant improvements.
Among the changes, which may be used individually or in combination, are (1) changes in the geometry of the capacitor discharge chamber; (2) the replacement of a tubular chamber for containing the meat with a xe2x80x9cdrum-headxe2x80x9d on which the meat sits and which is located at the upper end of the capacitor discharge chamber; (3) the provision of meat supporting structure which substantially holds the meat in place on the drum-head during capacitor discharge, and at least a portion of which optionally accompanies the meat through several stages of its movement; (4) a simplified indexing carrousel for delivering the meat to a location above the capacitor discharge chamber and for transporting the treated meat to a discharge location; (5) the more effective provision of a rarefaction or negative compression wave; and (6) the provision of certain improvements in tenderization involving subjecting the meat, especially boneless chicken breasts, to shock wave treatment in combination with other operations.